Abstract

The earliest experiments involving viruses were designed to separate them from microbes that could be seen in the light microscope and that usually could be cultivated on rather simple media. In the experiments that led to the first discoveries of viruses, by Beijerinck and Ivanovski (tobacco mosaic virus), Loeffler and Frosch (foot-and-mouth disease virus), and Reed and Carroll (yellow fever virus) at the turn of the century, one single physicochemical characteristic was measured, that being their small size as assessed by filterability (Waterson and Wilkinson, 1978). No other physicochemical measurements were made at that time, and most studies of viruses centered on their ability to cause infections and diseases. The earliest efforts to classify viruses, therefore, were based upon perceived common pathogenic properties, common organ tropisms, and common ecological and transmission characteristics. For example, viruses that share the pathogenic property of causing hepatitis (e.g., hepatitis A virus, hepatitis B virus, hepatitis C virus, yellow fever virus, and Rift Valley fever virus) would have been brought together as “the hepatitis viruses,” and plant viruses causing mosaics (e.g., cauliflower mosaic virus, ryegrass mosaic virus, brome mosaic virus, alfalfa mosaic virus, and tobacco mosaic virus) would have been brought together as “the mosaic viruses.”